This invention relates to an organic electroluminescent device and a panel therewith.
1. Description of the Related Art
An organic electroluminescent device (hereinafter, referred to as an organic EL device) is promising for its use as a self-light emitting type of flat-displaying device. An organic EL device is, unlike an inorganic EL device, which requires to be driven by alternating current and a higher voltage, and may be driven by DC and a lower voltage, and may be easily multicolored because a variety of organic compounds can be used.
Thus, it has been intensely studied in expectation of its application to, e.g., a color display.
When an organic EL device is applied to a color display, it is necessary to provide emission of three primary colors for light, i.e., red, green and blue colors.
Among these, there have been reported a number of examples of green emission. For example, known green devices include those using tris(8-quinolinol)aluminum (Applied Physics Letters 51, p.913 (1987)) or a diaryl amine derivative (Japanese Patent No. 2,686,418).
For an organic EL device emitting red light, for example, Japanese Patent No. 2,795,932 describes an organic EL device where a blue light is subject to wavelength conversion in a fluorochrome layer, while Japanese Patent Laid-Open Nos. HEI 07-272854, HEI 07-288184 and HEI 08-286033 have described an organic EL device emitting red light where a red fluorochrome is doped in a luminescent layer emitting a green or blue light.
For a blue luminescent device, there have been described many types of devices; for example, those using a stilbene compound (Japanese Patent Laid-Open No. HEI 05-295359), a triaryl amine derivative (Japanese Patent Laid-Open No. HEI 07-53955), a tetraaryl diamine derivative (Japanese Patent Laid-Open No. HEI 08-48656) and a styrylated biphenyl compound (Japanese Patent Laid-Open No. HEI 06-132080).
A blue luminescent material itself, however, has a larger energy gap (an energy difference between HOMO and LUMO levels) than a red or green luminescent material and thus has a higher ionization potential than an electron-transporting layer made of, for example, an 8-hydroxyquinoline metal complex such as tris(8-quinolinol)aluminum and bis(8-quinolinol) magnesium or an oxadiazole derivative, which are usually used in an organic electroluminescent device. Therefore, as shown in FIG. 4, the electron-transporting layer has poor hole-blocking property, so that holes may pass through the layer, leading to reduction in a hole-electron recombination efficiency. As a result, there have been problems such as a lower EL luminescence efficiency, a reduced maximum brightness and shift in a luminescent site.
Meanwhile, an organic EL device is, unlike an electric field-excitation luminescence type of inorganic EL device, a carrier injection type of device which emits a light by injecting hole carriers and electron carriers from an anode and a cathode, respectively, to recombine these carriers. For achieving such performance improvement in an organic EL device, it is believed that a laminate device in which a luminescent layer is combined with a charge transporting layer is more desirable than a monolayer device consisting of a luminescent layer alone. It is because an appropriate combination of a luminescent and a charge transporting materials in a multilayered device allows an energy barrier to be lowered during hole injection from an anode or electron injection from a cathode to facilitate charge injection as well as the charge transporting layer acts as a blocking layer inhibiting transmission of holes or electrons through the luminescent layer. It may improve a numerical balance between holes and electrons in the luminescent layer, resulting in efficient recombination and thus improvement in an EL luminescence efficiency. It is, therefore, a key for preparing a highly-effective organic EL device to improve hole or electron blocking property in an interface between the luminescent and the charge transporting layers.
For a red or green luminescent layer with hole transporting property, an energy gap of a luminescent material is relatively smaller. There are, therefore, many materials to be an electron transporting material with an ionization potential higher than the luminescent material. However, in a conventional blue organic EL device, a blue luminescent material has a larger energy gap as described above and it is difficult to select an electron transporting material which exhibits good deposition property and film-quality stability. As shown in FIG. 4, tris(8-quinolinol)aluminum (Alq3) is commonly used as an electron transporting or green luminescent material as an example.
This material exhibits good carrier transporting and deposition properties, but has an ionization potential of 5.67 eV, which is lower than that of a hole-transporting blue luminescent material, 5.7 to 5.8 eV. It, therefore, exhibits less hole-blocking property to a hole-transporting blue luminescent material, so that holes may pass through the material, leading to reduction in a recombination efficiency and an EL luminescence efficiency.
An objective of this invention is to ameliorate the problems in the above organic EL blue device, particularly to improve brightness and efficiency in a blue device within a range of BLUE, GREENISH BLUE and PURPLISH BLUE where an X,Y coordinate is below (0.25, 0.25) in the C.I.E. chromaticity diagram (1931).
In accordance with a first aspect of the present invention, there is provided an organic electroluminescent device comprising at least two organic-compound layers selected from the group consisting of a luminescent layer emitting a blue light, an electron injection layer and an electron transporting layer between heteropolar electrodes, wherein the electron injection or electron transporting layer has an ionization potential higher than that of the luminescent layer.
In accordance with a second aspect of the present invention, in the first aspect, the luminescent layer has hole transporting property.
In accordance with a third aspect of the present invention, in the first or second aspect, the electron injection or electron transporting layer comprises a compound represented by a general formula (1): 
where M represents a metal atom; R1 to R6, which may be different or the same, are a group independently selected from the group consisting of hydrogen atom, or halogen, and alkyl, alkoxy, and cyano group; L represents a ligand having a group selected from the group consisting of halogen and substituted or unsubstituted alkoxy, aryloxy, and alkyl; and n represents 1 or 2, provided that when n is 2, groups represented by the same symbol among R1 to R6 may be different or the same.
In accordance with a fourth aspect of the present invention, in any one of first to third aspects, the luminescent layer comprises a compound represented by a general formula (2) or (3): 
where R1 to R4 independently represent hydrogenatom, alkyl, alkoxy, aryl, aryloxy, amino, or cyano; and when n is more than one, groups represented by the same symbol among R1 to R4 on different rings may be different or the same; R5 and R6 independently represent an optionally substituted aryl with 6 to 12 carbon atoms; and n is an integer of 3 to 6; 
where M represents a metal atom; R11 to R19, which may be different or the same, independently represent a group selected from the group consisting of hydrogen, halogen, alkyl, alkoxy, aryl, and aryloxy; R11 to R14 or R15 to R19 may be combined together to form a saturated or unsaturated ring; Ra is a group which may be substituted as in R11 to R19, Rb is a hetero atom, and Ra and Rb may be independently or mutually combined to form a hetero ring; n represents 2 or 3; and R represents a divalent group.
In the present invention, it is preferred that the luminescent layer has an ionization potential of less than 5.9 eV.
In the present invention, it is preferred that the electron transporting layer has an ionization potential of 5.9 eV or more.
In the present invention, it is preferred that the electron transporting layer has an ionization potential at least 0.1 eV higher than that of the luminescent layer.
In the present invention, it is preferred that the electron transporting layer has a glass-transition temperature of 80xc2x0 C. or higher.
In accordance with another aspect of the present invention, there is provided a panel comprising the organic electroluminescent device in any one of the first to eighth aspects.